This invention relates, overall to an automated ultrasonic inspection system of mill rolls used in the process of metal fabrication, and, in particular to the use of creeping waves as the specific wave mode used for the detection of surface and near surface defects such as cracks, porosity, etc.
Mill rolls are an integral component of many metal fabricating processes. The rolling surface of a mill roll is typically subjected to high cyclic stresses created by direct loading and/or thermal cycles. The result is the formation of surface cracks that can quickly propagate to failure if not detected and removed. For this reason, mill rolls are periodically removed from service and placed in a roll grinding machine that mechanically removes a work hardened layer of material from the rolling surface. The rolls are then inspected to assure that all cracks are removed prior to reintroducing back into the mill.
The most common ultrasonic method for roll inspection is performed using a handheld Surface Wave (also called Raleigh Wave) ultrasonic transducer. This process involves applying couplant along the top of the roll and slowly moving the transducer down the roll axis while propagating sound in the circumferential direction. At the completion of this test, the roll is then turned 180 degrees and the procedure repeated. This manual exam was later developed into an automated technique where the transducer is mechanically held and scanned down the roll as the roll is rotated. However, automation of the Surface Wave technique for roll inspection never developed into a commercial success due to problems associated with water on the roll surface as described below. A serious complication to using Surface Waves for roll inspection is the sensitivity that this wave mode has to the presence of fluid, such as water drops, on the roll surface. A Surface Wave creates very small displacements in the direction perpendicular to the surface plane. As a result, any substance on the roll surface, such as a water drop, will be displaced as the wave front moves underneath it. This has two effects on the Surface Wave: 1) energy is removed from the wave front and 2) a reflected signal is produced that returns to the transducer from the water drop. The resulting reflected signal can appear identical to that produced by a surface breaking crack. Therefore, it is critical that all water, oil, etc, be removed from the roll surface prior to testing when using Surface Waves. This is made more difficult since cooling fluids are used during the grinding of the roll surface. In addition, conventional ultrasonic transducers require the use of fluid between the roll surface and the bottom of the transducer. This fluid, or couplant, is needed for transmission of sound between the roll and the transducer. During an automated scan where the roll is being rotated, this couplant layer will leave a trail of water that will interfere with the data taken on the subsequent rotation. U.S. Pat. No. 5,469,743 addresses this issue by proposing a wiper mechanism that eliminates this couplant material as well as water used to cool during grinding. This type apparatus has been found to be unreliable in practice since it cannot eliminate all surface water especially during grinding when water is splashed around the roll.
This patent describes an inspection system that uses an alternative ultrasonic method, Creeping Waves, instead of Surface Waves. Creeping Waves have been found to be highly sensitive to surface breaking cracks but not affected by surface contaminants such as water drops, oil, etc.
Ultrasonic Creeping Waves are produced by propagating high angle Longitudinal Waves into the mill roll at a location underneath the ultrasonic transducer. Since Creeping Waves are nothing more than high angle compression waves, the direction of particle displacements is always in the same direction as the direction of wave propagation. Therefore, no out of plane displacements are produced on the surface of the roll. Therefore, the sound wave is not affected by fluid on the roll surface making inspection possible during grinding processes without the need for water removal mechanisms.
The ideal system for use in the inspection of mill rolls is one that can be operated while the roll is being ground by the grinding machine. During the process of grinding, a large amount of water is sprayed on the roll at the location of the grinding stone. This water is required for cooling of the grinding wheel. This cooling fluid saturates the entire roll. The use of Surface Waves has been found to be impractical while grinding since the roll has to be completely dry in front of the ultrasonic transducer to avoid the possibility of a water drop reflection being interpreted as a surface crack. Surface Waves have been successfully implemented on mill rolls using electromagnetic acoustic transducers. However, this type of ultrasonic transducer does require that the mill roll be completely dried prior to inspection, thus making it impractical for use during the grinding process. This invention is unique since it permits the real-time ultrasonic inspection of a mill roll during the grinding process. With this capability, the operator can monitor the presence of the crack and halt grinding at the moment the crack is no longer detectable. This ability minimizes the amount of roll material removed while maximizing grinder utilization. Performing the roll inspection during grinding also eliminates the time required for roll drying and post grinding inspection.
A system based on the use of Creeping Wave transducers would appear very similar to a system using Surface Waves transducers. Both systems would require transport mechanism that lowers the transducers to the roll surface during testing. Both systems would require a couplant delivery system consisting of tubes providing water to transducer housings. Both systems could be manufactured to inspect for both circumferential defects and axially oriented defects. Two major differences exist between a Surface Wave system and a Creeping Wave system. First, the Creeping Wave system does not require any apparatus for water removal from the roll surface. Secondly, the Creeping Wave system can be used while grinding the roll. Mounting the transducer assembly at the same axial position as the grinding wheel will allow for real-time feed back to the grinder operator on defect removal status. Inspecting for defects during grinding allows the operator to terminate grinding once the defect is no longer detected. This minimizes grinding time and maximizes mill roll life, both a major advantage and cost savings for the mill. Current practice requires that the operator grind the mill roll until a predetermined amount of material has been removed from the roll. The roll is then dried and inspected manually or using an automated system. If a crack is found, the mill roll is ground some more and retested. This process is repeated until the roll is defect free.
The mill roll inspection system consists of the ultrasonic transducers, the couplant delivery system and the data acquisition system. The unique component of this system lies in the transducer used and more specifically, the Creeping Wave mode produced by this transducer. The use of this method of ultrasonic inspection permits the inspection of mill rolls while they are wet.
It is an object of the present invention to provide a method to ultrasonically inspect mill rolls using ultrasonic Creeping Waves.
It is another object of this invention to provide an ultrasonic inspection that does not require the drying of the roll surface in front of the transducer assembly prior to performing an inspection.
It is another object of this invention to provide a real-time ultrasonic inspection that can be performed during the grinding operation of a mill roll without the need to remove grinding cooling fluid.
It is still another object of this invention to provide an ultrasonic inspection of mill rolls that is sensitive to surface and near surface defects such as cracks and porosity, without being effected by surface contaminates such as oil or water.
These and other objects will become apparent when reference is had to the drawings.